336 BALANID.E. 



tubes, and, in the lower part, a striated (and sometimes serrated) ap- 

 pearance, from the exposure of the parallel, approximate plates, with 

 which the outer lamina of the shell is internally strengthened. In 

 only a few young specimens, the whole outer lamina of the shell was 

 well preserved ; and in these the surface was very smooth, and even 

 glossy, giving to the specimens a quite different aspect ; even in partially 

 corroded specimens, the lower part of the shell sometimes is quite 

 smooth. Generally, the radii are developed ; in most specimens they 

 are narrow, but sometimes of moderate width ; their summits are 

 oblique, and their edges often notched or toothed. The recipient 

 furrow, in each opposed compartment, is often almost as wide as the 

 radius itself, and is equally notched. In some much corroded specimens 

 there were no radii. Basal diameter of largest specimen, 1*1 of an inch. 



Scuta, generally thick, sometimes very thick, with the external sur- 

 face usually much corroded : articular furrow rather wide ; articular 

 ridge not very prominent ; adductor ridge prominent. The rostral 

 depressor muscle is attached in a small oblong pit, sometimes including 

 little crests ; and the lateral depressor muscle is attached to what may 

 be described either as three or four parallel furrows or crests. 



Terga, with the spur placed close to the basi-scutal angle, so that 

 there is no basal margin on that side ; spur short, with its lower end 

 truncated and rounded ; broad, even exceeding, when measured across 

 the upper part, half the width of the valve. Articular furrow wide. 

 Apex not beaked. 



Structure of Walls and Radii. — This species differs from all the others 

 of the genus in having only a single row (tig. 3 d) of parietal tubes ; these 

 are large, quadrangular, but elongated in the ray of the circle. They are 

 not tilled up, even at the very top of the shell, but they become thickly 

 lined all round with compact shelly matter. When the surface of the 

 shell is disintegrated, these upfilled tubes greatly affect, as already 

 stated, the external appearance. The outer lamina near the basis is 

 internally strengthened by longitudinal, sharp, approximate ridges or 

 plates, which, also, often affect, after corrosion, the external appearance. 

 The radii have their sutural edges formed by a set of narrow, branching 

 ridges or septa ; the ends of which, seen externally, often give a 

 notched outline to this edge ; the recipient furrows in the opposed 

 compartments are deep, and their edges likewise are often notched : 

 the interspaces between the branching ridges are filled up solidly. The 

 alee have their edges coarsely crenated. The lower edge of the sheath 

 is not free. 



The mouth and cirri present no particular characters : the third 

 cirrus has both its rami elongated, with the terminal segment tapering. 

 In the three posterior pairs of cirri, the tufts of little spines between 

 the main pairs are rather large. 



Affinities. — This species has no particular affinity with any other. 

 The circumstance of there being only a single row of parietal tubes is 

 not so important a difference as might at first be thought, inasmuch as 

 in the other species, during their quite early youth, the walls are 

 formed of only a single row of tubes or pores. 



